piston sizes of Stoptech ST-40 BBK:It's 34 mm and 38 mm.
So let's do the math:
Pi * radius² = area
908 mm² + 1134 mm² =
2043 mm²stock Ate FN3 54 caliper =
2290 mm²So the Stoptech ST-40 seems to be a good match.
Keep in mind
a BBK also provides better pad pressure distribution and larger rotor size.
Cayenne brakes aren't (there are three):
34/36/38 mm : 907.920 + 1017.876 + 1134.115 = ~
3060 mm²32/36/38 mm : 804.248 + 1017.876 + 1134.115 = ~
2956 mm²30/34/38 mm : 706.858 + 907.920 + 1134.115 = ~
2749 mm².... obviously Cayenne/Touareg/Q7 calipers WILL increase pedal travel (without a
much larger master cylinder) and weaken both brake force distribution, pedal feel
and booster/brake assist functionality. At least LHD Golfs/GTIs have a dual rate
booster and such significiently different pedal travel will shift its threshold.
BTW, S3/R32/R - FNRG 57 mm :
2552 mm²Don't get me wrong - shurely some other BBKs also have proper piston sizes,
but I bet many don't. Before buying a BBKs make sure it is a good match to your
existing system (master cylinder and rear calipers).